Method and system for producing LNG

ABSTRACT

A method is described for production of LNG from an incoming feed gas ( 1 ) on an onshore or offshore installation, and it is characterised by the following steps: 1) the feed gas is led through a fractionation column ( 150 ) where it is cooled a separated in an overhead fraction with a reduced content of pentane (C5) and heavier components, and a bottom fraction enriched with heavier hydrocarbons, 2) the overhead fraction from the fractionation column is fed to a heat exchanger system ( 110 ) and is subjected to a partial condensation to form a two-phase fluid, and the two-phase fluid is separated in a suitable separator ( 160 ) a liquid ( 5 ) rich in LPG and pentane (C3-C5) which is re-circulated as cold reflux to the fractionation column ( 150 ), while the gas ( 6 ) containing lower amounts of C5 hydrocarbons and hydrocarbons heavier than C5 is exported for further processing in the heat exchanger system ( 110 ) for liquefaction to LNG with a maximum content of ethane and LPG 3) the cooling circuit for liquefaction of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step. A system for carrying out the method is also described.

The present invention relates to a method for optimal production of LNGon a fixed or floating offshore installation, as can be seen in thepreamble of the independent claim 1.

The invention also relates to a system for implementing the methodcomprising a fractionation column for feeding feed gas, a heat exchangersystem for cooling down and partially condensing the overhead gas streamfrom the fractionation column, a separator for separation of thetwo-phase stream from the heat exchanger system, a device for return ofliquid from the separator to the fractionation column and feeding thisliquid to the upper part of the column as reflux, and a device forrouting the gas from the separator back to the heat exchanger system forfurther cooling and liquefaction to LNG.

The invention aims to use a closed gas expansion process to liquefy thenatural gas, and in that the gas is first fed through a fractionationcolumn where the gas is cooled and separated into an overhead fractionwith reduced content of pentane (C5) and heavier components, and abottom fraction enriched with the heavier hydrocarbons, furthermore, inthat the fractionation column reflux is generated as an integrated partof the system for liquefaction in that the overhead gas is partiallycondensed. By carrying out the liquefaction in accordance with theinvention, production of liquefied gas with maximum content of ethaneand LPG (liquid petroleum gas) is achieved at the same time as theefficiency of the gas expansion process is increased and theby-production of unstable/volatile liquid with a high content of ethane,LPG and pentane is minimised.

In particular, the invention comprises a method and a system forliquefaction of natural gas or other hydrocarbon gas from a gas field orfrom a gas/oil field, where it is appropriate to liquefy the gas tofacilitate transportation of the gas from the source to the market. Thisis particularly relevant for offshore oil/gas fields.

In this context, natural gas means a mixture of hydrocarbons where anessential part consists of methane. Natural gas is normally liquefied byconsiderably cooling down the gas such that it condenses and becomes aliquid. With LPG is meant liquid petroleum gas that encompasses propaneand butanes (C4, C4 components).

The aim of the invention is to render liquefaction of gas energyefficient at the same time as the process is kept simple so that theequipment can be used offshore, and then especially on floatinginstallations. By-production of condensate during the liquefaction isminimised and the efficiency is maximised (the need for fuel gas isminimised).

The method according to the invention is characterised by the followingsteps:

-   1) that the feed gas is led through a fractionation column (150)    where it is cooled and separated into an overhead fraction with    reduced content of pentane (C5) and heavier components, and a bottom    fraction enriched with heavier hydrocarbons,-   2) that the overhead fraction from the fractionation column is fed    into a heat exchanger system (110) and is subjected to partial    condensation to form a two-phase fluid, and the two-phase fluid is    separated in a suitable separator (160) to a liquid (5) rich in LPG    and pentane (C3-C5) which is re-circulated as cold reflux to the    fractionation column (150), while the gas (6) containing lower    amounts of C5 hydrocarbon and hydrocarbons heavier than C5, is    routed for further treatment in the heat exchanger system(110) for    liquefaction to LNG with maximum content of ethane and LPG, and-   3) that the cooling circuit for liquefaction of gas in the heat    exchanger system comprises an open or closed gas expansion process    with at least one gas expansion step.

The preferred embodiments of the method are defined in the dependentclaims 2-10.

The system according to the invention is characterised in that thecooling system which is used for cooling, condensing and liquefaction ofthe gas in the heat exchanger system comprises an open or closed gasexpansion process with at least one gas expansion step. The system ispreferably designed and configured to separate the feed gas so that theoverhead gas stream of the system will be enriched with the majority ofthe butane (C4) and hydrocarbons with a lower normal boiling point thanbutane, and the bottom product of the fractionation column will beenriched with most of C6 and components with a normal boiling pointhigher than C6.

BACKGROUND

Liquefaction of natural gas can be carried out with the use of a gasexpansion process, where a cooling medium goes through a processingcircuit based on compression, cooling, expansion and thereafter heatexchange with the fluid that is to be cooled down. For example, forliquefaction of natural gas, one can use a compressed cooling medium ingas phase, normally nitrogen or methane, which is pre-cooled andthereafter expanded across an expansion valve or a turboexpander. Thegas expansion generates very cold gas, or a mixture of gas and liquid,which is then used for liquefaction of natural gas and to pre-cool thecompressed cooling medium gas. The gas expansion processes arerelatively simple and therefore very well suited to offshoreinstallation. However, the processes have a somewhat lower efficiencythan the more advanced processes, such as, for example, mixedrefrigerant cycle processes, and thus require much compression equipmentand much energy.

In order to produce LNG it is normally required that the gas has arelatively high content of methane. However, most of the feed gases willalso contain some heavier hydrocarbons such as ethane, propane, butane,pentane, etc. Some requirements with respect to the content of heavierhydrocarbons in the liquid gas are normally present:

The specific energy content per cubic meter of liquefied gas mustnormally not exceed given sales specifications.

The content of pentane (C5) and upwards, and also aromatic compounds ofthe liquid gas, must be kept below defined limits to avoid freeze out inthe cooling process.

The simplest way to limit the content of heavier hydrocarbons in theliquid gas is to partially condense the gas and then separate thecondensed liquid from the gas, which is further cooled for liquefaction.The separation is normally carried out as an integrated part of the cooldown process, typically at a temperature between 0° C. and −60° C.Separated condensate can be heated up again as a part of the coolingprocess to utilise the refrigeration potential.

In large land based LNG installations (so called “base load”installations) most of the propane and heavier hydrocarbons are normallyremoved and in many cases also a considerable part of ethane, before oras a part of, the liquefaction. This is done to meet the salespecifications and to be able to produce and sell the valuable ethane,LPG and condensate/naphtha. Comprehensive processes are normally usedwith low temperature fractionation columns both as a part of the cooldown process and as separate units outside the cooling system.

However, for offshore LNG production it is undesirable to handleproducts other than the liquid natural gas. Where oil or condensate isalso produced one can however permit separation of condensate forstabilisation and export together with another oil and/or condensate.However, stabilised condensate will, in the main, consist of C6+ with arelatively low content of pentane and lighter components. Hydrocarbonslighter than C6 can generally not be stored or transported safelywithout being cooled down or being under pressure. Some separatedhydrocarbons or condensate can be used as fuel, but beyond that it isdesirable to retain these components in the LNG product. Due to smallerLNG volumes and the possibility for later blending into large LNGvolumes, it can be appropriate offshore to produce a liquid natural gaswith a considerably higher, and preferably a maximum, content of heavierhydrocarbons.

The present invention represents a considerable optimisation forapplication offshore, and especially on a floating unit, in that arelatively simple and robust gas expansion process is used forliquefaction of natural gas, and in that the energy efficiency of thisprocess is increased at the same time as the amount of liquid gas ismaximised by maximising the content of ethane and LPG, at the same timeas the amount of hydrocarbons heavier than methane which is separatedout as bi-products in the liquefaction process is minimised.

An installation which comprises the system according to the inventioncan thereby simply be adapted and be installed, for example, on boardfloating offshore installations where space is often a limiting factor.

References to Known Technology and Other Publications, and Comparisonswith the Present Invention:

Initially reference is made to EP-1.715.267 which describes a methodwhich includes natural gas being cooled and being led through afractionation column where it is separated into an overhead fraction anda bottom fraction. The bottom fraction is enriched with heavierhydrocarbons and is exported out of the system. The overhead fraction iscooled and forms a two-phase fluid which is separated in a separator.The liquid phase is re-circulated to the fractionation column whilst thegas phase is fed further to a heat exchanger system. Cooling of theoverhead fraction is carried out with a free standing cooler. The EPpatent consequently describes a classical and well-known distillationprocess.

Furthermore, the set-up is standard practice in so-called “base load”LNG installations, where both cooler 5 and cooler 11 (ref. figures inthe EP patent) are parts of the pre-cooling installation of the plant,which is normally carried out as a multistep propane coolinginstallation. However, the set up in the EP patent does not integrate afractionation column and a downstream LNG condensation process as oneaims with the present invention. Integration is here meant that twosystems are tightly connected together and function as one system andthat material streams and/or energy streams are flowing both waysbetween the systems.

The refrigeration work which according to EP-1.715.267 cools theoverhead fraction and generates so-called reflux to the fractionationcolumn, comes according to the description not from the same coolingcircuit that carries out further cooling and condensation of the naturalgas, but apparently from an external cooling process.

International patent application WO-2005/071333 describes a well-knowndouble gas expansion which is used to liquefy boil off gas from storagetanks for LNG. In practice, such boil off gases contain only methane andnitrogen.

Patent publications US2006/0260355 A1 and U.S. Pat. No. 6,662,589describe systems which apparently are similar to the present invention,but which in reality are considerably different from the presentinvention. The systems in the referred publications comprise processesfor simultaneous liquefaction of natural gas and recovery/separation ofcomponents heavier than methane, i.e. ethane and heavier components,where ethane, LPG and heavier components are fractionated into salesproducts and where the liquid gas has a considerably reduced content ofethane and heavier components. This is achieved by leading the feed gasto a fractionation column where it is contacted with an ethane richreflux such that the fractionation column separates the feed into anoverhead gas fraction with a considerably reduced content of componentsheavier than methane and a liquid stream from the bottom considerablyenriched with components heavier than methane. The ethane rich reflux isgenerated in that the gas from the fractionation column is partiallycondensed, and in addition by cooling down and condensing a stream richin ethane which is re-circulated from a fractionation train forfractionation of the bottom fraction from the fractionation column.

Patent publications U.S. Pat. No. 6,401,486, U.S. Pat. No. 6,742,358 andWO02006/115597 A2 describe systems for simultaneous liquefaction ofnatural gas and recovery/separation of components heavier than methane,i.e. ethane and heavier components. The processes themselves are alsoconsiderably different from and more complex than the present inventionin that the feed gas is first cooled down in, amongst others, the heatexchanger(s) for liquefaction of gas, and also by heat exchange with aflash expanded separated liquid and with fluid from the bottom of thecolumn. Furthermore, the whole or part of the feed gas stream isexpanded through a turboexpander or a Joule-Thompson expansion valvebefore it is led to the fractionation column.

The patent publications US 2006/0260355 A1, U.S. Pat. No. 6,662,589,U.S. Pat. No. 6,401,486 and also U.S. Pat. No. 6,742,358 consequentlyrelate to processes to minimise the content of ethane, LPG and also theheavier hydrocarbons in the liquid gas, whilst the present inventioncomprises a system and a method to maximise the content of methane,ethane and LPG in the liquid gas. None of the US patent application2006/0260355 A1, U.S. Pat. No. 6,662,589, U.S. Pat. No. 6,401,486 orU.S. Pat. No. 6,742,358 describe the increase in energy efficiency whichcan be achieved for a gas expansion process with the integratedseparation column that receives a reflux rich in C3-C5 from theliquefaction heat exchanger(s) for production of LNG.

A process is described in DE patent 10205366 for simultaneousliquefaction of natural gas and recovery/separation of componentsheavier than ethane, and where separated LPG and heavier components arefractionated to sales products. This is achieved by first partiallycooling down the feed gas in the condensation installation forliquefaction of natural gas and then by leading the cooled down feed gasto a fractionation column where it comes into contact with a reflux richin ethane so that the fractionation column separates the feed into anoverheard gas fraction with a considerably reduced content of componentsheavier than ethane, and a liquid stream from the bottom considerablyenriched with components heavier than ethane. The reflux rich in ethaneis generated in that the gas from the fractionation column is partiallycondensed and thereafter brought into contact with a C4/C5 stream in asecond fractionation column, and where the C4/C5 fraction isre-circulated from a fractionation train for fractionation of the bottomproduct from the first fractionation column. DE patent 10.205.366comprises, in other words, a process to minimise the content of LPG ofthe liquid gas, and also the heavier hydrocarbons, while the presentinvention comprises a system and a method to maximise the content of LPGin the liquid gas. The publication DE 10.205.366 does not describe anincrease in energy efficiency which can be achieved in a gas expansionprocess with the integrated separation column which receives a refluxrich in C3-C5 from the liquefaction heat exchanger(s) for production ofLNG.

In U.S. Pat. No. 4,690,702 an LNG process is described where the feedgas is firstly pre-cooled in the cooling installation for LNGproduction, thereafter to be fed to a first fractionation column whereit is brought into contact with a cooled ethane rich reflux that isre-circulated from a second fractionation column for fractionation ofthe bottom stream from the first column. The publication does notcomprise a system where a reflux rich in C3-C5 for a fractionationcolumn is achieved by partially condensing the overhead gas product fromthe fractionation column as an integrated part of an LNG process.

U.S. Pat. No. 7,010,937 shows a system for simultaneous liquefaction ofnatural gas and recovery / separation of components heavier thanmethane. According to this publication the gas feed is pre-cooled andpartially condensed so that a liquid stream can be separated in aseparator and where this liquid stream is fractionated in a firstfractionation column to generate an overhead gas which is cooled down toproduce a reflux for a second fractionation column. The gas flow fromthe separator is expanded across a gas expander and fed to the secondfractionation column. Therefore this US patent has little in common withthe present invention as it is defined in the subsequent claims.

DESCRIPTION OF THE INVENTION

The invention will now be described in more detail with reference to theenclosed figures in which:

FIG. 1 shows a principal embodiment with main components and mainfunctionality.

FIG. 2 shows the invention with an alternative embodiment.

FIG. 3 shows the invention with an alternative embodiment that includesfurther stabilisation of the heavier hydrocarbons that are separated out(condensate).

FIG. 4 shows the invention in detail carried out by using a double gasexpansion process.

FIG. 5 shows the invention carried out by using a hybrid cooling circuitwith a gas expansion loop and a liquid expansion loop.

FIG. 6 shows an example of a hot temperature curve and a coldtemperature curve (composite curve) for a conventional nitrogenexpansion cycle.

FIG. 7 shows an example of a hot temperature curve and a coldtemperature curve (composite curve) for a nitrogen expansion cycleobtained by using the present invention.

FIG. 8 shows a comparison of the curves shown in the FIGS. 6 and 7.

With reference to FIG. 1 the system for optimised liquefaction of gascomprises, as a minimum, the following principle components:

-   -   an incoming gas stream 1 which shall be cooled down and        liquefied,    -   a fractionation column 150 in which the incoming gas is cooled        and is separated into an overhead fraction 2 with a reduced        content of pentane and heavier components,    -   a bottom fraction 3 enriched with the heavier hydrocarbon        components,    -   a system of heat exchangers 110, in which the incoming gas is        cooled down and partially condensed for separation of heavier        hydrocarbons, and further cooling and liquefaction,    -   a product stream 11 comprising cooled liquefied gas,    -   a product stream 3, which mainly comprises pentane and heavier        hydrocarbons, and    -   a cooling system for cooling and liquefaction of the gas        comprising a gas phase cooling medium stream 20, at least one        cycle compressor 100, at least one aftercooler 130, at least one        gas expander 120.

Incoming and cleaned feed-gas 1, for example, a methane rich hydrocarbongas, is first fed to a fractionation column 150, where the gas is cooleddown in contact with a colder reflux fluid. During the cooling andcounter current contact with the colder fluid, the feed gas is separatedinto an overhead fraction 2 with a reduced content of the hydrocarbonsthat have a molecular weight higher than pentane (C5), and a bottomfraction 3 enriched with C6 and hydrocarbons that have a highermolecular weight than C6. The overhead fraction 2 from the fractionationcolumn is then led to the heat exchanger system 110, where the gas iscooled down and partially condensed so that the resulting two-phasefluid 4 can be separated in a suitable separator 160. A liquid 5 rich inLPG and pentane (C3-C5), which is separated in the separator 160, isre-circulated as cold reflux to the fractionation column 150. (Note: Herer det feil i originalteksten (150/160) men det bø r vaere opplagt forbehandlende instans at dette er skrivefeil og at meningen ikkeforandres) As this fluid is generated by condensation by cooling, thereflux liquid 5 will have a lower temperature than the feed gas 1. Thegas 6 from the separator 160 has now further reduced its content of C5hydrocarbons and hydrocarbons higher than C5. This gas is then led backto the heat exchanger system 110 for further cooling, condensation andsub cooling. The liquefied gas 11 is alternatively led through a controlvalve 140 that controls the operating pressure and flow through thesystem.

In a preferred embodiment the gas feed stream 1 is pre-cooled by asuitable external cooling medium such as available air, water, seawateror a separate suitable refrigeration system/pre-cooling system. For thelatter external cooling method, a separate closed mechanicalrefrigeration system with propane, ammonia or other appropriaterefrigerant is often used.

In a preferred embodiment the fractionation column 150 and the separator160 are operated at pressures and temperatures such that the completesystem (the fractionation column 150 and reflux separator 160) generatea component split/separation point in the normal boiling point area(NBP) between −120° C. and 60 C. This can, for example, correspond tothe light key component for the separation being butane (C4) with anormal boiling point between −12° C. and 0° C., and the heavy keycomponent being a C6 component with a boiling point between 50° C. and70° C. The overhead gas stream 6 of the system will then be enrichedwith most of the butane (C4) and hydrocarbons with a lower normalboiling point than butane. The bottom product 3 from the fractionationcolumn will be enriched with most of C6 and components with a normalboiling point higher than C6, while pentane (C5, NBP=28-36° C.) is atransition component which is distributed in the gas product of thesystem and the bottom product from the fractionation column.

Cooling and condensation of the feed gas in the heat exchanger system110 is provided by a closed or open gas expansion process. The coolingprocess starts in that a cooling medium 21 comprising a gas or a mixtureof gases (such as pure nitrogen, methane, a hydrocarbon mixture, or amixture of nitrogen and hydrocarbons), at a higher pressure, preferablybetween 3 and 10 MPa, is fed to the heat exchanger system 110 and cooledto a temperature between 0° C. and −120° C., but such that the coolingmedium stream is mainly a gas at the prevailing pressure and temperature31. The pre-cooled cooling medium 31 is then led into a gas expander 121where the gas is expanded to a lower pressure between 5%-40% of theinlet pressure, but preferably to between 10% and 30% of the inletpressure, and such that the cooling agent mainly is in the gas phase.The gas expander is normally an expansion turbine, also calledturboexpander, but other types of expansion equipment for gas can beused, such as a valve. The flow of pre-cooled cooling agent is expandedin the gas expander 121 at a high isentropic efficiency, such that thetemperature drops considerably. In certain embodiments of the invention,some liquid can be separated out in this expansion, but this is not arequirement for the process. The cold stream of cooling agent 32 is thenled back to the heat exchangers 110 where it is used for cooling andalternatively condensing of the other incoming warm cooling mediumstreams and the gas that shall be cooled, condensed and sub cooled.

After the cold cooling medium stream 32 has been heated in the heatexchanger system 110, the cooling medium will exist as the gas stream51, which in a closed loop embodiment is recompressed in an appropriateway for recycle, and is cooled with an external cooling medium, such asair, water, seawater or an appropriate refrigeration unit.

Alternatively, the cooling system in an open embodiment will use acooling medium 21 consisting of a gas or a mixture of gases at a higherpressure received from an appropriate source, for example, from the feedgas that is to be treated and cooled down. Furthermore, the openembodiment comprises that the low pressure cooling medium stream 51 isused for other purposes or, in an appropriate way, is recompressed to bemixed with the feed gas that is to be treated and cooled down.

In a preferred embodiment, the returning cooling medium stream 51 is ledfrom the heat exchanger 110 to a separate compressor 101 driven by theexpansion turbine 121. In this way, the expansion work is utilised, andthe energy efficiency of the process is improved. After the compressor101, the cooling agent is cooled further in a heat exchanger 131, beforethe stream is further compressed in the cycle compressors 100. The cyclecompressors 100 can be one or more units, alternatively one or morestages per unit. The cycle compressor can also be equipped with intercooling 132 between the compressor stages. The compressed cooling medium20 is then cooled by heat exchange in an aftercooler 130 with the helpof an appropriate external cooling medium, such as air, water, seawateror a suitable separate refrigeration cycle, to be re-used as acompressed cooling medium 21 in a closed loop.

In a preferred embodiment, the system of heat exchangers 110 is one heatexchanger which comprises many different “warm” and “cold” streams inthe same unit (a so-called multi-stream heat exchanger).

FIG. 2 shows an alternative embodiment where several multi-stream heatexchangers are connected together in such a way that the necessary heattransfer between the cold and warm streams can be accomplished. FIG. 2shows a heat exchanger system 110 comprising several heat exchangers inseries. However, the invention is not related to a specific type of heatexchanger or number of exchangers, but can be carried out in severaldifferent types of heat exchanger systems that can handle the necessarynumber of hot and cold process streams.

FIG. 3 shows an alternative embodiment where the fractionation column150 is equipped with a reboiler 135 to further improve the separation (asharper split between light and heavy components), and also to reducethe volatility of the bottom fraction in the column. This can be used todirectly produce condensate which is stable at ambient temperature andatmospheric pressure.

FIG. 4 shows in details the invention applied in a more advancedembodiment where a double gas expansion process is used. In thisembodiment, the compressed cooling medium stream 21 is first cooled downto an intermediate temperature. At this temperature, the cooling agentstream is divided into two parts, where one part 31 is taken out of theheat exchanger and is expanded in the gas expander 121 to a low pressuregas stream 32. The other part 41 is pre-cooled further to be expanded inthe gas expander 122 to a pressure essentially equal to the pressure instream 32. The expanded cold cooling agent streams 32, 42 are returnedto different inlet locations on the heat exchanger system 110 and arecombined to one stream in this exchanger. Heated cooling agent 51 isthen returned to recompression. In an alternative embodiment to thesystem in FIG. 3, the compressed cooling agent stream 20 in the doublegas expansion circuit can be split into two streams before the heatexchanger 110 to be cooled down to different temperatures in separateflow channels in the heat exchanger 110.

The same applies for the heating of the returned cold cooling agentstreams 32, 42. The embodiment is otherwise in accordance with FIG. 3.

FIG. 5 shows in detail the invention carried out with the use of ahybrid cooling loop where one cooling medium is used both in a pure gasphase and in a pure liquid phase. In this embodiment a closed coolingloop provides the cooling of the feed gas in the heat exchanger system110. The Said cooling cycle starts by methane or a mixture of methaneand nitrogen, where methane makes up at least 50% of the volume, beingcompressed and aftercooled to a compressed cooling medium stream 21, andwhere this cooling medium stream is pre-cooled, and at least a part 31of the cooling medium stream is used in the gas phase in that it isexpanded across a gas expander 121 and that at least a part 41 of thecooling agent stream is condensed to liquid and is expanded across avalve or liquid expander 141.

It is emphasised that the embodiment of the invention is not limited tothe cooling processes described above only, but can be used with any gasexpansion cooling process for liquefaction of natural gas or otherhydrocarbon gas, where the cooling is mainly achieved by using one ormore expanding gas streams.

By carrying out the liquefaction of the natural gas in accordance withthe invention, a product of liquefied gas is produced which has amaximum content of methane, ethane and LPG, however, at the same timedoes not contain more than the permitted level of pentane and heavierhydrocarbons with a normal boiling point above 50-60° C. At the sametime, the by-production of volatile hydrocarbons with considerablecontent of ethane, propane and butane is minimised or eliminated, wheresuch will be difficult to handle on an offshore installation for LNGproduction. At the same time more liquid natural gas will also beproduced with lower energy consumption than for similar cooling cyclesconfigured without the fractionation column which receives cold andLPG-rich reflux from the cooling down process. (Note: Her er detskrivefeil i originaltekst)

The reason for the energy consumption for the gas expansion processesfor liquefaction of the natural gas is being reduced with the use of theinvention compared to a similar cooling process without the integratedseparation column has several aspects:

The heavier hydrocarbons which are essential to separate out to preventfreezing during the liquefaction will be condensed and be separated atconsiderably higher temperatures than in conventional methods, in thatmuch of the condensing takes place in the fractionation column. Thisreduces the energy loss in the cooling process in that cooling load ismoved to a higher temperature range.

The heat exchanger system 100 of the cooling process receives the gaswhich is to be liquefied as stream 2 (the overhead gas stream in thefractionation column), which has a reduced temperature with respect tothe actual gas feed stream 1. A gas expansion process is characterisedin that the warm and cold cooling curves are dominated by the largeamount of gas which is used as cooling medium. These gas streams formlinear cooling curves. The reduced feed temperature into the heatexchanger results in a “break point” on the warm cooling curve (the sumof the streams which are being cooled), so that it is possible to obtaina general reduction of the distance between the warm and cold coolingcurves. This provides a better temperature adaption, reduced energy lossand thus a reduced energy consumption to drive the cooling process.

Preliminary analyses and comparisons show that necessary compressor workper kg liquid natural gas which is produced can be reduced by 5-15% fora gas expansion cycle carried out in accordance with the inventioncompared to conventional methods.

FIG. 6 shows warm and cold cooling curves (warm and cold compositecurves, i.e. the sum of all warm streams that are to be cooled down andthe sum of all cold streams that are to be heated up, respectively) forthe heat exchanger system 110 carried out in accordance with the presentinvention, and with a double nitrogen expansion process as coolingsystem. FIG. 7 shows corresponding warm and cold cooling curves for acorresponding cooling process with the same feed, but carried out in aconventional way without the fractionation column. The curves appear tolook alike, but by considering FIG. 8, which shows a section and boththe systems in (Note: skrivefeil i originaltekst) the same curve, the“break point” and the better adaption can clearly be seen.

Example

The example below shows natural gas with 90.4% methane by volume whichis to be liquefied, where the invention is used to maximise the amountof liquid gas and at the same time minimise the by-production ofunstable hydrocarbon liquid with a high content of ethane, propane andbutane. The stream data refer to FIG. 1, 2, 3, 4 or 5.

Stream No. 1 2 3 4 5 6 11 Gas fraction 1.00 1.00 0.00 0.95 0.00 1.000.00 Temperature 40.0 19.2 35.9 −20.0 −20.0 −20.0 −155.0 (° C.) Pressure2740 2738 2745 2725 2730 2723 2655 (kPa abs) Mole flow 4232 4422 44 4422235 4185 4185 (kmol/h) Mass flow 78980 87539 3410 87539 11969 7554175541 (kg/h) Mole fraction (%) Nitrogen 0.51% 0.49% 0.02% 0.49% 0.03%0.52% 0.52% Methane 90.4% 87.4% 11.8% 87.4% 19.5% 91.3% 91.3% Ethane4.38% 4.53% 2.58% 4.53% 6.84% 4.40% 4.40% Propane 2.29% 2.95% 4.17%2.95% 15.04%  2.27% 2.27% i-Butane 0.68% 1.25% 2.80% 1.25% 11.92%  0.65%0.65% n-Butane 0.66% 1.52% 3.79% 1.52% 17.30%  0.62% 0.62% i-Pentane0.17% 0.70% 2.52% 0.70% 10.57%  0.14% 0.14% n-Pentane 0.17% 0.79% 3.61%0.79% 12.49%  0.12% 0.12% n-Hexane 0.44% 0.32% 43.62%  0.32% 6.25% 0.02%0.02% n-Heptane 0.19% 0.00% 18.29%  0.00% 0.02% 0.00% 0.00% n-Octane0.055%  0.000%  5.187%  0.000%  0.000%  0.000%  0.000%  n-Nonane 0.014% 0.000%  1.339%  0.000%  0.000%  0.000%  0.000%  n-Decane+ 0.002% 0.000%  0.214%  0.000%  0.000%  0.000%  0.000% 

1. Method for production of LNG from an incoming feed gas (1),characterised by the following steps: a) the feed gas is led through afractionation column (150) where it is cooled and separated into anoverhead fraction (2), with a reduced content of the hydrocarbons thathave a molecular weight higher than pentanes (C5) and enriched with mostof the butane (C4) and hydrocarbons with a lower normal boiling pointthan butane, and a bottom fraction (3) wherein hydrocarbons having amolecular weight from hexanes (C6) and higher of the feed gas areconserved. b) the overhead fraction of the fractionation column is ledinto a heat exchanger system (110) and is subjected to a partialcondensation to form a two-phase fluid, and the two-phase fluid isseparated in a suitable separator (160) into a fluid (5) rich in LPG andpentane (C3-C5) which is re-circulated as cold reflux to thefractionation column (150), while the gas (6), enriched with most of thebutane (C4) and hydrocarbons with a lower normal boiling point thanbutane, is reintroduced into the heat exchanger system (110) forliquefaction to LNG consisting essentially of methane, ethane, propaneand butane, and wherein c) the cooling circuit for liquefaction of gasin the heat exchanger system comprises an open or closed gas expansionprocess with at least one gas expansion step.
 2. Method according toclaim 1, characterised in that the fractionation column (150) and theseparator (160) are operated at pressures and temperatures which lead tothe complete system (the fractionating column 150 and reflux separator160) generating a component split/separation point in the normal boilingpoint range (NBP) between −12° C. and 60° C.
 3. Method according toclaims 1-2, characterised in that the light key component for theseparation is butane (C4) with a normal boiling point between −12° C.and 0° C., and the heavy key component is a C6 component with a boilingpoint between 50° C. and 70° C., whereby the overhead gas stream (6) ofthe system will contain the most of comprising a considerably reducedcontent of n-butane and hydrocarbons with a lower normal boiling pointthan n-butane, and the reject stream (3) of the system comprises most ofC6 and components with a normal boiling point higher than C6.
 4. Methodaccording to claims 1-3, characterised in that the fractionation column(150) and the separator (160) are operated so that pentane (C5,NBP=28-36° C.) is a transitional component that is distributed both inthe overhead gas stream (6) of the system and the reject stream (3) ofthe system.
 5. Method according to one of the preceding claims,characterised in that the temperature of the feed gas is reduced throughthe fractionation column (150) so that the temperature of the gas whenit is fed into the heat exchanger system (110) is lower than thetemperature of the cooling gas stream at the hot end of the heatexchanger system (hot pinch point temperature).
 6. Method according toone of the preceding claims, characterised in that a reboiler (135) isconnected to the fractionation column (150) to reduce the steam pressureof the bottom product.
 7. Method according to one of the precedingclaims, characterised in that the heat exchanger for liquefaction (LNGproduction) comprises one or more multi-stream heat exchangers. 8.Method according to one of the preceding claims, characterised in thatit is carried out with a closed gas expansion process with at least onenitrogen expander.
 9. Method according to one of the preceding claims,characterised in that it is carried out with a closed hybrid coolingprocess with methane/nitrogen as a cooling agent, where the coolingagent is used both in the gas phase and in the liquid phase, and wherethe cooling circuit has at least one gas expander and at least oneappliance for expansion of a liquid cooling agent.
 10. Method accordingto one of the preceding claims, characterised in that it is carried outwith an open gas expansion process with at least one gas expander, inwhich a suitable gas at a higher pressure is used as cooling gas, andwhere the expanded gas at a lower pressure is not recompressed forrecycling but is used for another purpose.
 11. System for carrying outthe method according to claims 1-10 comprising a fractionation column(150) for feeding in a feed gas, a heat exchanger system (110) forcooling down and partially condensing the overhead gas stream of thefractionation column, a separator (160) to separate the two-phase streamfrom the heat exchanger system, set up to recycle fluid from theseparator to the fractionation column and import this fluid to the upperpart of the column as a reflux, and appliance to lead the gas from theseparator back to the heat exchanger system for further cooling down andliquefaction to LNG, characterised in that the cooling system which isused for cooling down, condensing and liquefying of gas in the heatexchanger system comprises an open or closed gas expansion process withat least one gas expansion step.
 12. System according to claim 11,characterised in that the system is designed and configured to separatethe feed gas such that the overhead gas stream (6) of the system will beenriched with most of the butane (C4) and hydrocarbons with a lowernormal boiling point than butane, and the bottom product in thefractionation column will be enriched with most of the C6 and componentswith a normal boiling point higher than C6.
 13. An optimized gasliquefaction system of the gas expansion type for the production of LNGfrom an incoming natural gas stream, characterised in that the systemcomprises: a) an open or closed gas liquefaction circuit comprising agaseous refrigerant, at least one gas expander for cooling therefrigerant by gas expansion, and one or more heat exchangers for heatexchange between the natural gas, LNG and refrigerant streams, b) afractionation column arranged for receiving the incoming natural gasprior to introduction of the natural gas into the liquefaction circuit,said fractionation column being further arranged to cool and separatethe incoming natural gas into an overhead gaseous fraction, with areduced content of the hydrocarbons that have a molecular weight higherthan pentanes (C5) and enriched with most of the butane (C4) andhydrocarbons with a lower normal boiling point than butane, and a bottomliquid fraction wherein hydrocarbons having a molecular weight fromhexanes (C6) and heavier are conserved, and arranged to lead theoverhead gaseous fraction to at least one of the heat exchangers of theliquefaction circuit, whereby the overhead feed gas fraction is cooledto a two-phase fluid, and c) a separator arranged for receiving thetwo-phase fluid and separating the two-phase fluid into a gas componentwherein hydrocarbons having a molecular weight from butanes and lighterare conserved, and a liquid component rich in C3-C5 hydrocarbons, andfurther arranged to return the liquid component back to thefractionation column as a cold reflux liquid and to return the gascomponent to the liquefaction circuit for further cooling andcondensation to LNG.
 14. (canceled)
 15. (canceled)
 16. A systemaccording to any of claim 13, wherein the liquefaction circuit comprisesthe gaseous refrigerant at an inlet pressure of 3-10 MPa being fed tothe heat exchanger or system of heat exchangers and cooled to atemperature between 0 and −120 deg C., and further wherein the cooledgaseous refrigerant is expanded to a pressure between 5% and 40% of theinlet pressure, and then being led back to the heat exchanger or systemof heat exchangers to provide cooling.
 17. A system according to any ofclaim 13 or 16, wherein the liquefaction circuit comprises two expansionstages, wherein the gaseous refrigerant at an inlet pressure of 3-10 MPais split in two parts either before or after pre-cooling, and where theparts are pre-cooled to different temperatures before expansion toessentially the same lower pressures and led back to the heat exchangeror system of heat exchangers to provide cooling.
 18. A system accordingto any of claim 13 or 16-17, wherein cooling in the fractionation columnis essentially provided by the reflux liquid from the separator.
 19. Asystem according to any of claim 13 or 16-18, wherein a reboiler (135)is connected to the fractionation column (150) to reduce the vapourpressure of the bottom product.
 20. A system according to any of claim13 or 16-19, wherein the overhead gas fraction is cooled into thetwo-phase fluid by being in heat-exchanging, counter-currentrelationship with the gaseous refrigerant in one or more heat exchangersof the liquefaction circuit.
 21. A system according to any of claim 13or 16-20, wherein the liquefaction circuit comprises one or moremulti-stream heat exchangers configured in series or parallel, or both.22. A system according to any of claim 13-22, wherein the gas expanderessentially isentropically cools the refrigerant.
 23. A system accordingany of claim 13 or 16-23, wherein the liquefaction circuit comprises aclosed gas expansion process with two or more gas expansion stages foressentially isentropically cooling the refrigerant by gas expansion, andwhere the refrigerant inlet temperature for the second gas expanderstage is lower than the refrigerant inlet temperature for the first gasexpander stage.
 24. A system according to either any of claim 13 or16-24, wherein the gaseous refrigerant comprises nitrogen gas, or amixture of nitrogen and hydrocarbons.